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EventWaitHandle.GetAccessControl Method

Gets an EventWaitHandleSecurity object that represents the access control security for the named system event represented by the current EventWaitHandle object.

Namespace:  System.Threading
Assembly:  mscorlib (in mscorlib.dll)

'Declaration
Public Function GetAccessControl As EventWaitHandleSecurity

Return Value

Type: System.Security.AccessControl.EventWaitHandleSecurity
An EventWaitHandleSecurity object that represents the access control security for the named system event.

ExceptionCondition
UnauthorizedAccessException

The current EventWaitHandle object represents a named system event, and the user does not have EventWaitHandleRights.ReadPermissions.

-or-

The current EventWaitHandle object represents a named system event, and was not opened with EventWaitHandleRights.ReadPermissions.

NotSupportedException

Not supported for Windows 98 or Windows Millennium Edition.

ObjectDisposedException

The EventWaitHandle.Close method was previously called on this EventWaitHandle.

The GetAccessControl method uses the following combination of flags (combined using the bitwise OR operation) to search for permissions: AccessControlSections.Access, AccessControlSections.Owner, and AccessControlSections.Group.

The user must have EventWaitHandleRights.ReadPermissions rights to call this method, and the event must have been opened with the EventWaitHandleRights.ReadPermissions flag.

The following code example demonstrates the cross-process behavior of a named system event with access control security. The example uses the OpenExisting(String) method overload to test for the existence of a named event.

If the event does not exist, it is created with initial ownership and access control security that denies the current user the right to use the event, but grants the right to read and change permissions on the event.

If you run the compiled example from two command windows, the second copy will throw an access violation exception on the call to OpenExisting(String). The exception is caught, and the example uses the OpenExisting(String, EventWaitHandleRights) method overload to wait on the event with the rights needed to read and change the permissions.

After the permissions have been read, using the GetAccessControl method, and changed, the event is opened with the rights required to wait on it and signal it. If you run the compiled example from a third command window, the example runs using the new permissions.

Imports System
Imports System.Threading
Imports System.Security.AccessControl

Friend Class Example

    <MTAThread> _
    Friend Shared Sub Main()
        Const ewhName As String = "EventWaitHandleExample5" 

        Dim ewh As EventWaitHandle = Nothing 
        Dim doesNotExist as Boolean = False 
        Dim unauthorized As Boolean = False 

        ' The value of this variable is set by the event 
        ' constructor. It is True if the named system event was 
        ' created, and False if the named event already existed. 
        
        Dim wasCreated As Boolean 

        ' Attempt to open the named event. 
        Try 
            ' Open the event with (EventWaitHandleRights.Synchronize 
            ' Or EventWaitHandleRights.Modify), to wait on and  
            ' signal the named event. 
            '
            ewh = EventWaitHandle.OpenExisting(ewhName)
        Catch ex As WaitHandleCannotBeOpenedException
            Console.WriteLine("Named event does not exist.")
            doesNotExist = True 
        Catch ex As UnauthorizedAccessException
            Console.WriteLine("Unauthorized access: {0}", ex.Message)
            unauthorized = True 
        End Try 

        ' There are three cases: (1) The event does not exist. 
        ' (2) The event exists, but the current user doesn't  
        ' have access. (3) The event exists and the user has 
        ' access. 
        
        If doesNotExist Then 
            ' The event does not exist, so create it. 

            ' Create an access control list (ACL) that denies the 
            ' current user the right to wait on or signal the  
            ' event, but allows the right to read and change 
            ' security information for the event. 
            
            Dim user As String = Environment.UserDomainName _ 
                & "\" & Environment.UserName
            Dim ewhSec As New EventWaitHandleSecurity()

            Dim rule As New EventWaitHandleAccessRule(user, _
                EventWaitHandleRights.Synchronize Or _
                EventWaitHandleRights.Modify, _
                AccessControlType.Deny)
            ewhSec.AddAccessRule(rule)

            rule = New EventWaitHandleAccessRule(user, _
                EventWaitHandleRights.ReadPermissions Or _
                EventWaitHandleRights.ChangePermissions, _
                AccessControlType.Allow)
            ewhSec.AddAccessRule(rule)

            ' Create an EventWaitHandle object that represents 
            ' the system event named by the constant 'ewhName',  
            ' initially signaled, with automatic reset, and with 
            ' the specified security access. The Boolean value that  
            ' indicates creation of the underlying system object 
            ' is placed in wasCreated. 
            '
            ewh = New EventWaitHandle(True, _
                EventResetMode.AutoReset, ewhName, _
                wasCreated, ewhSec)

            ' If the named system event was created, it can be 
            ' used by the current instance of this program, even  
            ' though the current user is denied access. The current 
            ' program owns the event. Otherwise, exit the program. 
            '  
            If wasCreated Then
                Console.WriteLine("Created the named event.")
            Else
                Console.WriteLine("Unable to create the event.")
                Return 
            End If 

        ElseIf unauthorized Then 

            ' Open the event to read and change the access control 
            ' security. The access control security defined above 
            ' allows the current user to do this. 
            
            Try
                ewh = EventWaitHandle.OpenExisting(ewhName, _
                    EventWaitHandleRights.ReadPermissions Or _
                    EventWaitHandleRights.ChangePermissions)

                ' Get the current ACL. This requires  
                ' EventWaitHandleRights.ReadPermissions. 
                Dim ewhSec As EventWaitHandleSecurity = _
                    ewh.GetAccessControl()

                Dim user As String = Environment.UserDomainName _ 
                    & "\" & Environment.UserName

                ' First, the rule that denied the current user  
                ' the right to enter and release the event must 
                ' be removed. 
                Dim rule As New EventWaitHandleAccessRule(user, _
                    EventWaitHandleRights.Synchronize Or _
                    EventWaitHandleRights.Modify, _
                    AccessControlType.Deny)
                ewhSec.RemoveAccessRule(rule)

                ' Now grant the user the correct rights. 
                ' 
                rule = New EventWaitHandleAccessRule(user, _
                    EventWaitHandleRights.Synchronize Or _
                    EventWaitHandleRights.Modify, _
                    AccessControlType.Allow)
                ewhSec.AddAccessRule(rule)

                ' Update the ACL. This requires 
                ' EventWaitHandleRights.ChangePermissions.
                ewh.SetAccessControl(ewhSec)

                Console.WriteLine("Updated event security.")

                ' Open the event with (EventWaitHandleRights.Synchronize  
                ' Or EventWaitHandleRights.Modify), the rights required 
                ' to wait on and signal the event. 
                '
                ewh = EventWaitHandle.OpenExisting(ewhName)

            Catch ex As UnauthorizedAccessException
                Console.WriteLine("Unable to change permissions: {0}", _
                    ex.Message)
                Return 
            End Try 

        End If 

        ' Wait on the event, and hold it until the program 
        ' exits. 
        
        Try
            Console.WriteLine("Wait on the event.")
            ewh.WaitOne()
            Console.WriteLine("Event was signaled.")
            Console.WriteLine("Press the Enter key to signal the event and exit.")
            Console.ReadLine()
        Catch ex As UnauthorizedAccessException
            Console.WriteLine("Unauthorized access: {0}", _
                ex.Message)
        Finally
            ewh.Set()
        End Try 
    End Sub  
End Class

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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